Porites matrices grow in the Bravo Crater in this handout photo made available April 14, 2008. Coral is again flourishing in the crater left by the largest nuclear weapon ever detonated by the United States, 54 years after the blast on Bikini Atoll, marine scientists said on Tuesday.

Credit: Reuters/Australian Research Council/Handout

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CANBERRA (Reuters) - Coral is again flourishing in the crater left by the largest nuclear weapon ever detonated by the United States, 54 years after the blast on Bikini Atoll, marine scientists said on Tuesday.

A team of research divers visited Bravo crater, ground zero for the test of a thermonuclear weapon in the remote Marshall Islands on March 1, 1954, and found large numbers of fish and coral growing, although some species appeared locally extinct. "I didn't know what to expect, some kind of moonscape perhaps. But it was incredible," Zoe Richards, from Australia's James Cook University, told Reuters about the team's trip to the atoll in the south Pacific.

"We saw communities not too far from any coral reef, with plenty of fish, corals and action going on, some really striking individual colonies," she said.

The 15 megatonne hydrogen bomb was 1,000 times more powerful than the blast which destroyed Hiroshima, vaporizing islands with temperatures hitting 99,000 Fahrenheit, and shaking islands even up to 124 miles away.

The resulting 4 mile-wide fireball left a crater 1 mile across and 80 yards deep, while the mushroom cloud rose 62 miles over the South Pacific and radioactive fallout reached Australia and Japan.

Richards, from the Australian government-backed Centre of Excellence for Coral Reef Studies, said the research team from Germany, Italy, Hawaii, Australia and the Marshall Islands found corals up to 9 yards high and some with 12 inch-thick trunks.

"It was fascinating. I've never seen corals growing like trees outside of the Marshall Islands," Richards said.

While above-water areas remained contaminated and unfit for human habitation, healthy sub-sea species probably traveled on strong winds and currents from nearby Rongelap Atoll, which was not bombed in a series of 23 tests between 1946-58.

"It is absolutely pristine for another tragic reason. It received fallout and was evacuated of people, so now underwater it's really healthy and prevailing winds have probably been seeding Bikini Atoll's recovery," Richards said.

Compared with a study made before the atomic tests, the team established that 42 species were missing compared to the early 1950s, with at least 28 of those locally extinct.

The team was asked by Marshall Islands authorities to investigate Bikini for the first time since the tests, in part to see if a small diving industry could safely be expanded.

The waters around Bikini are littered with wrecks of old , decommissioned ships sunk during the atomic tests, including the aircraft carrier USS Saratoga and the former Japanese flagship HIJMS Nagato, from which Admiral Yamoto gave the order to attack Pearl Harbour.

Richards said the ability of Bikini's corals to bounce back from "a single huge destructive event" was proof of their resilience, although that did not mean the threat to corals from climate change had been overestimated.

"Climate change is an ongoing struggle to survive with coral, with no reprieve in sight," she said. "After the atomic blasts they had 50 years undisturbed to recover."